In the drilling of oil, gas and disposal wells, the first part of the hole drilled into the earth is typically through a section where fresh water formations are exposed in the well bore. It is typically necessary to cement a string of surface pipe through any fresh water section in order to protect fresh water aquifers from contamination from drilling fluids and reduce problems that would otherwise occur later in the drilling operation if there were no surface pipe.
In areas where many highly prolific aquifers below surface pipe are encountered, drilling below surface pipe is accompanied by the circulation of drilling mud through the well bore. An ancillary benefit of surface pipe, in mud drilling, is to seal off unconsolidated formations which might take drilling mud and thereby create a blow out hazard. In other areas, typically where there are fewer aquifers below surface pipe and where the drilled formations are hard, such as limestones, dolomites, hard shales and thoroughly cemented sandstones, drilling is accompanied by the circulation of air through the well bore. Air drilling is much preferred in these areas primarily because of relatively high penetration rates and secondarily because of the ease of evaluating permeable formations for oil or gas. Accordingly, drilling with mud is widespread in the Gulf Coast, in the Gulf of Mexico and much of the mid-continent while air drilling is much preferred in Appalachia and areas of West Texas and other hard rock areas.
A major difficulty in air drilling is contending with the influx of water from permeable formations. There are essentially two solutions: increasing the rate of air flow through the drill string or adding soap to the air stream and thereby changing from straight air to foam, or some combination of more air and foam. It may accordingly be difficult, impractical or overly expensive under some circumstances to drill the surface hole with air because one is inherently contending with permeable aquifers that give up significant amounts of water. Thus, in air drilling country, it is often most efficient to drill the surface hole of an oil, gas or disposal well with mud, cement a string of surface pipe in the well bore and then convert to air drilling below surface pipe. This is not inexpensive but, all things considered, is the most efficient current approach under many circumstances. The alternative is to drill the surface hole with air and have enough air compressor capacity and/or foam capacity to keep the well bore unloaded regardless of the amount of water produced from near surface aquifers.
The problem of contending with produced water is made worse when it is necessary or desirable to drill surface holes that are large when compared to the O.D. of the drill string. The reason is the annulus between the well bore and the drill string is large and the rate of air flow is not sufficient to keep water droplets moving upwardly in the annulus. In these situations, the droplets coalesce and the upward air velocity is insufficient to keep the annulus unloaded with reasonable air flows thereby requiring the addition of surfactant to the air stream to generate foam.
One situation where it is necessary to drill large diameter surface holes is in the drilling of relatively deep wells which require running strings of large diameter surface casing in order to reach total depth with a well bore of sufficient diameter to accommodate a conventionally sized production string. One area where conditions dictate the drilling of large diameter surface holes in hard rock country is drilling wells to produce from the Marcellus or Utica shales in the northeastern United States. A similar set of circumstances exists in some areas of West Texas and New Mexico. Those skilled in the art will recognize that these conditions occur in many other locales.
In hard rock areas of this type, one occasionally encounters fractures, even in rocks near earth's surface. It has occurred in the past that drilling a large diameter surface hole with air and foam has intersected a fracture in a fresh water aquifer of a thoroughly cemented sandstone. This has resulted in delivering a large amount of soapy water into the fracture. The aquifer may be contaminated in very short order because of the high conductivity of the fracture. Because it is very difficult or impossible to predict the existence of near surface fractures in the intended path of a well bore, surprises of serious complications can occur in drilling surface holes with air and soap.
Standard air drilling is sometimes called direct air drilling where air is delivered downwardly through the drill string, out through the bit or hammer and then upwardly in the annulus between the drill string and the wall of the well bore. This is the technique that is universally used in the United States to drill oil, gas and disposal wells of any depth with air.
Reverse air drilling is also known where the drill string includes inner and outer concentric pipe strings, air is delivered downwardly through the annulus between the pipe strings, through the bit and then upwardly through the inner pipe string. Reverse air drilling is typically used in the United States to explore for expensive minerals where the goal is to recover all of the drill cuttings without contamination by well bore debris or without contact between drill cuttings and the wall of the well bore. A discussion of mud and air reverse drilling is found in Chapter 22 of The Transfer of Technology and in a publication of similar name by John L'Espoir published in Water Well Journal of November 2010.